Angelo Olivieri

Drag reduction by microbubbles in a turbulent boundary layer

An experimental characterization of the turbulent boundary layer over a flat plate in the presence of small amounts of microbubbles is performed. The average diameter of the injected bubbles is comparable with the local Kolmogorov lengthscale, and the bulk void fraction C¯ is approximately 0.1%. The velocity field of the liquid phase, as well as the bubble characteristics, is acquired by optical techniques. Even at the small void fraction typical of this investigation, the interaction between microbubbles and turbulence leads to significant modifications of the underlying flow field. In accordance with previous investigations, the main global effect consists in a reduction of the magnitude of the viscous drag. This has been checked with preliminary tests conducted in a towing tank, as well as by inferring the wall stress from the boundary layer velocity profile measured in a laboratory facility at the same conditions. Here, the local wall stress is found to drop by approximately 25%. An analysis of the tu...

Scars and Vortices Induced by Ship Bow and Shoulder Wave Breaking

Experimental data are provided for physical understanding and computational fluid dynamics (CFD) validation for the surface combatant David–Taylor model basin Model 5415 bow and shoulder wave breaking. A photographic study was conducted using 5.72m replica and 3.05m geosim models of Model 5415 over a range of Froude numbers (Fr) to identify Fr and scale effects on wave breaking and choose the best Fr for the local flow measurements, which include near- and far-field means and rms wave elevation and mean velocity under the breaking waves. The larger model and Fr=0.35 were selected due to the large extents of quasisteady plunging bow and spilling shoulder wave breaking. A direct correlation is shown between regions of wave slope larger than 17deg and regions of large rms in wave height variation. Scars characterized by sudden changes in the mean wave height and vortices induced by wave breaking were identified. Complementary CFD solutions fill the gaps in the relatively sparse measurements enabling a more complete description of the bow and shoulder wave breaking and induced vortices and scars. The combined results have important implications regarding the modeling of the bubbly flow around surface ships, especially for bubble sources and entrainment.

Statistical approach for estimating intervals of certification or biases of facilities or measurement systems including uncertainties

A statistical approach for estimating intervals of certification or biases of facilities or measurement systems including uncertainties is set forth based on M×N-order level testing, which is defined as M repetitions of the same N-order level experiment in M different facilities or in the same facility with M different measurement systems. In the absence of reference values, the mean facility or measurement system is used for assessing intervals of certification or biases. Certification or biases of facilities or measurement systems are defined as processes for assessing probabilistic confidence intervals for facilities or measurement systems for specific tests, data reduction equations, conditions, procedures, and uncertainty analysis. Similarly, subgroup analysis is performed for isolating and assessing levels of differences due to use of different model sizes (scale effects) or measurement systems. An example is provided for towing tank facilities for resistance tests using standard uncertainty analysis procedures based on an international collaboration between three facilities. Although the number of facilities are at a minimum, the results demonstrate the usefulness of an approach and support recommendation of future collaborations between more facilities. Knowledge of intervals of certification or biases is important for design, accrediting facilities or measurement systems, and CFD validation.

Effects of Surface Tension in the Near Field Wave Breaking of Ships

When model experiments are performed the viscous and surface tension forces are not scaled accordingly. Thus not all of the features of the flow can be satisfactorily reproduced at model scale. A comparative set of experiments for measuring the model resistance, the free surface elevation and the flow velocity in the near field, have been carried out for models of different scales for evaluating the influence of the dimensions in reproducing the complete wave breaking dynamics. The resistance curves of the models show that the scale effect is present both for low and high speeds. Comparison of the averaged surface elevation reveals that the largest model possess already some of the full scale features. The comparison of the flow velocity fields highlights substantial differences among the models in the formation of the vortical structures. The influence of these vortices on the free surface is discussed and a correlation with surface scars is proposed.Copyright